Intermittent Internet Connection since installation

I self-installed the Comcast router 2 days back at my home. The installation was simple and after successful installation, I was happy to see a speed of 300+mbps.

However, the intermittent connection issue problem started right after that. Every 2 minutes the internet goes out for 30 seconds and then comes back. When I check the speed, it shows 300+mbps before dropping to 0 (no connection). It has been happening continuously for the past 2 days.

I have gone through the app-based troubleshooting 4 times but to no avail (along with restarting the Gateway nth times). I even checked the coaxial cables and all connections are tight and sturdy. 

Given that I am using a brand-new Comcast modem and cables, equipment doesn't seem to be a problem.

My ATT (which I had at the same address) never had such problems even a single time in the past 1 year. Honestly, I am repenting why I went ahead with Comcast! 

Can someone pls guide more troubleshooting steps here? I would like to give Comcast a chance before deciding to eventually go back to ATT.